Aquifer Mixing Exercises

  1. A scientist studying water and pesticide movement above a shallow aquifer measures pesticide solution concentrations of 20 mg L-1 just above the water table. The aquifer has a porosity of 30% and it is 4 m thick. Annual recharge to the aquifer is estimated to be 300 mm per year. The pesticide is a new product so it does not exist in the aquifer. The critical concentration of interest is the USEPA Health Advisory Level of this pesticide which is 15 mg L-1. Answer the following questions, assuming the aquifer can be represented by the simplified model used here.

 

    1. How long will it be until the concentration in the aquifer reaches the critical value?
    2. If the recharge rate is less than 300 mm per year, will the time required increase or decrease? How long would it take if the recharge rate were 200 mm per year? How long would it take if the recharge rate is 400 mm per year?
    3. How much water enters the aquifer during the time required to reach the critical concentration for each of the recharge rates used in part c.? What do these results suggest? Can you test your conclusion for other recharge rates?
    4. What volume of water was in the aquifer underlying an area of 1 hectare? What height of water does this represent? What aquifer properties were required to answer this question? If the aquifer porosity were 40%, what aquifer thickness would be needed to keep the volume of water underlying the 1 ha area a constant. If these aquifer properties are used in the model, how long will it take to reach the critical concentration? Does this answer differ from that found in step a?
    5. If the critical concentration is 12 mg L-1, how long will it take to reach that concentration? What if the critical concentration is 18 mg L-1 ? Make a table of time to reach the critical concentration as the critical concentration changes from 1 to 20 mg L-1. Draw a graph of these results. Describe your findings.
    6. Examine the impact of the concentration of water entering the aquifer upon the time required to reach the critical concentration. Describe your findings in detail. Include graphs and tables to support your discussion. Make note of any special cases you discover.

 

  1. An aquifer has been used to supply drinking water for a city. It has been discovered that the water contains nitrate N at a concentration of 100 mg L-1. The critical concentration of nitrate N in groundwater is 10 mg L-1. Due to improved water and fertilizer management practices, the concentration of nitrate N entering the aquifer is only 5 mg L-1. If the recharge rate is 300 mm per year and the aquifer has a thickness of 6 m and a porosity of 25%,
    1. How long will it take to reduce the concentration to the critical level?
    2. What is the concentration at the end of 1, 2, 3, … years?